A fuel injector conventionally comprises a needle that is driven so as to open and close as a function of the pressure prevailing in a control chamber, which pressure depends on the position of a solenoid control valve. The small movements are carried out at high speed, and consistently enhanced performance is now requiring feedback of information regarding the actual position of the needle for optimal control. Devices are known in which a sensor is arranged on the injector, or an injector in which the surfaces of the components of the body are electrically insulated so that an electrical resistance measurement can be carried out between two elements of the body of the injector. Complex and expensive devices have not yet proven their industrial viability, and there is a need to provide a simple and effective device.